Exoantigens are produced by pathogenic salivarian and nonpathogenic stercorarian trypanosomes such as Trypanosoma brucei and T. lewisi, respectively. These antigens elicit a protective, trypanocidal response, but one of the exoantigens of T. lewisi is ablastinogen, which elicits the ablastic response, a reproduction-inhibiting immunity that is unique to and responsible for the benign nature of stercorarian infections. It is proposed to continue studies of exoantigens and other associated components on the cell surface of these trypanosomes. These will include isolation of ablastinogen by affinity chromatography with recently collected ablastic IgG, and after characterization of the antigen, a search for a counterpart in T. brucei; continuation of studies of the avidity of ablastin and of immune complex formation with exoantigens during infection; isolation and characterization of the two variant specific antigens (exoantigens) of T. lewisi; continuation of studies of cap formation with all identified surface ligands of T. lewisi; a study of surface coat formation and its possible relation to antigenic variation in T. brucei by indirect immunofluorescence and capping techniques; and an analysis of the cell surface of antigenic variants of T. brucei for shared, accessible antigens that are not ordinarily immunogenic. Through this analytical approach, it may be possible to learn why an ablastic response fails to develop against the pathogenic African trypanosomes. Moreover, the proposed study will provide basic information about the biology of these parasites and may reveal significant components that could provide the basis for effective control measures against the pathogenic trypanosomes.